DIY Anamorphic Lens

What is an Anamorphic Lens

When you watch a movie, you've probably noticed those black bars along the
top and bottom of the image. Movies are produced in various "aspect ratios"
(the ratio of the width:height of the image). Some movies are produced
in 16:9 aspect ratio. If you watch these movies on an older non-HD television
(which has a 4:3 aspect ratio), then you will get black bars above
and below the image. If you have a newer HDTV with a native 16:9 "wide
screen" then you won't get any black bars since the aspect ratio of the HDTV
matches the aspect ratio of the movie. However, some movies are produced
in 2.35:1 "Cinema Scope" format. When
watching a 2.35:1 movie, even on your widescreen HDTV, you will still get black
bars on the top and bottom of the image.

Currently, there are no televisions or projectors that have a native 2.35:1
aspect ratio. But there are several ways to remove these black bars to
get the full Home Cinema Scope experience.

First, of course, you need a 2.35:1 aspect ratio screen. You can buy
screens in this aspect ratio, or make one yourself. Some people just
paint a large wall or piece of material that hangs on the wall. In my
case, I stretched "black out" fabric over a large screen-door frame. You
can buy the metal strips that go around the edge of a screen-door at the hardware
store. Just mount these strips to a large sheet of plywood, then stretch
the black-out fabric just as if it were screen-door mesh. It makes a
great screen.

Once you have a wide enough screen, the easiest way to eliminate the black
bars is to zoom your projector so that the black bars are projected above the
top of the screen and below the bottom of the screen. Essentially, you
just zoom out until the width of the image fills the width of your screen. If
you have a dark-colored wall, or cover it with black felt, then you'll never
notice the light-spill over the top and bottom of the screen.

I used this method for years and it worked fine. The problems with this
method is that you are enlarging the entire image, making the pixels larger,
and are wasting the brightness and resolution from the black bars which are
still being projected above and below your screen. Also, newer projectors
often cannot zoom an image enough to fill the screen, and might also require
lens shift adjusting to center the image vertically on your screen.

The solution to this, and the best way to achieve Home Cinema Scope, is using
a special lens in front of the projector called an "Anamorphic Lens". An
"Anamorphic Lens" will stretch (or compress) an image in one dimension (vertically
or horizontally). Most projectors have a "zoom mode" that stretches the
image vertically and eliminates the black bars. Rather
than projecting "black" and wasting those pixels, all vertical pixels are used
for the movie image itself. This
vertical stretch will cause people and objects on the screen to look tall and
skinny. However,
once the projector is displaying the full image vertically, you can then use
an Anamorphic Lens to stretch the image horizontally to restore the correct
aspect ratio.

That's a lot of words! Let's look at some pictures to understand this
better:

Normal 2.35:1 Movie on a 16:9 widescreen HDTV

This is what you are probably used to seeing on your HDTV. Those annoying
black bars on the top and bottom. Now let's project this onto a 2.35:1
screen:

Normal 2.35:1 Movie on a 2.35:1 Cinema Scope screen

(click for larger image)

Yes, I can hear you already: "This is even worse!" Now you
have black bars on the top and bottom AND on the left and right! Patience...this
is just the beginning. Now, let's activate the projector mode that stretches
the image to fill the entire vertical height of the screen. On my Infocus
Screenplay 7205 projector, this mode is called "Letterbox":

2.35:1 Movie on a 2.35:1 screen in Letterbox "stretch" mode

(click for larger image)

This stretch is just coming from the projector's "Letterbox" mode. No
lens is used yet. Notice that C3PO and R2D2 look stretched (tall and
skinny). This is what the lens will fix. When an Anamorphic Lens
is placed in front of the projector, we get this final image:

Now that's looking nice! No black bars at all. And since the full
vertical resolution of the projector is being used, the image remains nice
and bright and sharp. Static images really only provide part of the story. I
can't convey the emotional immersion that results in watching the full Cinema
Scope image rather than the first image that only utilized the middle of the
screen. It feels like the same difference between watching a movie on
TV vs watching it in the theater.

Of course, the downside to using a 2.35:1 screen is that when you watch 16:9
format material (or regular 4:3 format television), then you have black bars
on the left and right of the image instead of the top and bottom. This
kind of setup is called a "Constant Image Height" (CIH) because the height
of the image remains the same no matter what you are watching. This is
exactly how normal movie theaters work. The image is always the same
height, but the theater has retractable curtains on the left and right side
of the screen that they can use to adjust the width of the screen depending
upon the aspect ratio of the movie being shown. You can add curtains
to your own home theater to provide the same feature. In my case, I just
live with the black bars on the left and right. But masking the left and right
bars with curtains is much easier than masking the top and bottom black bars
in a normal HDTV setup.

Making an Anamorphic Lens using Prisms

Most people who want a Home Cinema Scope setup simply purchase a commercial
Anamorphic Lens, or purchase a high-end projector that already has such a lens
attached. But these lenses typically cost over $1000. You might
think that it would be very difficult to make your own Anamorphic Lens. You'll
be surprised to learn that making your own Anamorphic Lens is actually a very
easy DIY project (even easier than making your own 2.35:1 screen).

The
secret is "prisms". If you remember back to your high
school science class, a prism is often used to split a beam of light into a
rainbow (think of the cover of the "Dark Side of the Moon" by Pink
Floyd). Whenever a beam of light hits a surface, some light is reflected, and
some light is "refracted" (or bent). Notice that as the beam of white light
hits the surface of the prism, each color is refracted (bent) a different amount.
Then, when each color hits the second edge of the prism (on the right), each
color is bent even more.

Now, imagine putting two prisms next to each other, but reversed so that the
second prism takes the rainbow from the first prism and converts it back into
a white beam of light. With two prisms, you put a white beam in, and
you get a white beam out. But by adjusting the angles of the prisms relative
to each other, you can bend the outgoing beam of light and essentially magnify
it. This is exactly how an Anamorphic Lens works!

Anamorphic Lens using 2 Prisms

The prisms shown in the above diagram more closely match the prisms that are
commercially available in large sizes. Remember that the image coming from your
projector starts with a small size of just a couple inches. But when the light
beam reaches the screen, it is much larger. The beam of light increases in size
as you get further from the projector. So these prisms need to be several inches
in size to handle the full beam from the projector.

Where to get Prisms

There are several ways to get a prism the size that you need. Many people
have made their own prisms by cutting glass to make the surfaces, then seal
the glass together to make a hollow prism, and then fill it with water or mineral
oil. You can make very large prisms in this way. However, most home
theater owners don't like the idea of potentially leaking prisms hanging from
the ceiling in front of their projector. Sealing these glass prisms so
that they do not leak over a long period of time can be difficult.

Fortunately,
an inexpensive solution is available. Several companies
now manufacture crystal "award" plaques. These crystal plaques
are meant to be engraved with some award and then sit on your desk or a shelf. They
are wedge-shaped, with the base of the plaque larger than the top of the plaque. If
you look at the side profile of these crystal wedges, they match the picture
of the prisms shown above. They come in several sizes, such as 4"x6" (measuring
the front face that you would normally have engraved) up to 6"x7.5".

There are several sources for these prisms. In the U.S., I have used Massillon
Plaque. In Australia you can use EvRight. If
you call your local award plaque company, they can probably point you to other
sources. At the time that I purchased the small prisms, they were about
$29 each (without engraving). Be sure to call the company to find out if
blank wedges are available. Also, the blank wedges are usually cheaper
than the prices listed on the web site, since they don't need to charge you for
any engraving.

These are not plastic or acrylic. They are very heavy, crystal glass. I
don't think they are "lead crystal", but are some modern variation. I've
even heard some people say that it's the same stuff they make spacecraft windows
out of. What's important is that they are optically very clear and the
surfaces are very flat and uniform. It's important to get two identical
prisms, since the second prism needs to cancel the rainbow created by the first
prism. If there are differences in the prism angles, or if the surfaces
are not perfectly flat, then you will get more "chromatic aberration" (which
will look like blurring at the edges of the screen).

Initial Testing

If you are planning to build your own Anamorphic Lens, I recommend that you
just purchase a couple of prisms and then start playing with them. It's
impossible to provide exact construction dimensions and angles because each
projector setup will be slightly different. The angle between the two
prisms will depend upon the throw distance of your projector, and the size
of your screen. So, if possible, just put your projector on a table and
place the prisms in front of the projector. Then start playing with the
angles. In my case, I placed the prisms on a piece of paper, and when
I had the prisms adjusted, I just traced the outline of the prisms onto the
paper as a template for construction (more on that later).

Just set up the prisms in roughly the configuration shown in the 2-prism diagram
above. As you adjust the angle of the first prism, you will be adjusting
the right-edge of the image on the screen. As you adjust the angle of
the second prism, you will be adjusting the left-edge of the screen. I
placed some tape on my screen (the blue painters tape that doesn't leave any
marks or residue) and make marks for the proper 2.35:1 size. Then I just
adjusted the prism angles until the image expanded to hit the marks.

In fact, it's so easy to set this up and align it, that it's even possible
to use these prisms for temporary setups where you just put the projector on
the coffee table, then place the prisms in front of it. It's not perfect,
but it works easier than you might think. But after a few minutes of
playing with the prisms you will get the hang of it. The initial results
are so impressive, that you might be tempted to just start watching movies
already! Trust me...you'll want to watch your entire collection of 2.35:1
movies all over again!

Building an Enclosure

Playing with the prisms on the coffee table is fun. But eventually you
are going to want to mount these prisms in some type of enclosure that you
can place in front of your projector. In my case, my projector hangs
from the ceiling. So I needed an enclosure that could also hang from
the ceiling. The type of enclosure that you need will really depend a
lot on where you have your projector mounted already.

Because we are building a "horizontal stretch" lens, you do not need to move
your current projector. Some people like to make a "vertical compression"
lens (which you can also make using two prisms) and then move the projector
so that it projects the full width of the 2.35:1 image, and then use the lens
to compress the image vertically to fit on the screen. Some people feel
that you get a better quality image using vertical compression, but I didn't
want to move my existing projector location, and I also wanted a way to move
the lens out of the way when I'm not watching 2.35:1 movies.

Enclosures can be built out of a variety of materials. The most common
materials seem to be either aluminum or wood, depending upon your preference
(and what tools you have available). I made my own enclosure out of 1/2"
MDF because it is cheap and easy to work with. My design was based upon the
original "Aussiemorphic
Lens" from Mark Techer. His blog provided my initial inspiration. Ideas
from many people in the DIYAudio.com
forum were also used. In particular, I wanted to be able to easily adjust
the prism angles once they were inside the enclosure for fine tuning then lens.

To
start, I needed to attach bolts to the top and bottom of the prism which would
be used as rotation points. Just gluing a flat-head bolt to the
prism doesn't work very well. However, if you first cut a triangle of
plexiglas (or other material...some have just used cardboard), then you can
drill a hole in the plexiglas, countersink it for the screw head, then place
the screw between the prism and the plexiglas and epoxy the entire piece of
plexiglas to the prism.

Remember that the "top and bottom" that we are gluing screws to are really
the left and right edges as shown in the picture of the award plaque sitting
on a desk. When placed in front of the projector beam, these become the
top and bottom edges within the enclosure. The screws fit into holes
in the top and bottom of the enclosure with wing nuts. Loosening the
nuts allow the prisms to be rotated to adjust the alignment.

It is also very important to mask the sides of the prisms. As I mentioned
earlier, whenever a beam of light hits a surface, part is refracted (bent),
but part is reflected. With these glass prisms, a reflection from one
surface is only about 4% of the main beam intensity. A double-reflection
is only about 0.1% of the main beam intensity. However, in a dark theater,
even a 0.1% reflection is visible on the screen. Most of these reflections
exit from the two ends of the prisms. I just used black electrical tape
to mask the ends.

Once each prism has a bolt plate glued to the top and bottom, then it's time
to make the enclosure itself.

The enclosure is shown from the top (upper-left picture), with the projector
at the top and the screen towards the bottom. The bottom-left diagram
is looking from the screen towards the projector through the box. The
enclosure really is a fairly simple 4-sided box. As shown in the bottom-left
view, the top is screwed to the sides, while the bottom is glued and held to
the sides using splines (or biscuits if you have a biscuit-joiner). I
tried to minimize screws and only use them where it was necessary, since the
top needs to be removed to change or clean the prisms. But MDF doesn't
hold screws very well, so biscuit joints and glue work best.

The top view also shows the tentative position of the prisms (looking down
on the prisms). The circles represent the screws that were glued to the
top and bottom of the prisms. The inside of the enclosure is lined with black
felt that is glued to the MDF using a spray adhesive (rubber cement also works
well).

Once the enclosure is built, then you need to decide how to mount it in front
of your projector. In my case, my projector is flush mounted with the
ceiling. Since I wanted to be able to remove the lens from the front
of the projector, I designed a "sled" which attaches to the ceiling via two
drawer slides. The sled itself is just another piece of MDF, painted
white to match the ceiling. To attach the enclosure box to the sled,
I made a U-shaped bracket using oak. This bracket needs to be stronger
than MDF, so a hardwood like oak or maple would work well. An aluminum
bracket would also work well. The bracket is slotted to allow adjustment
on the sled, and also adjustment to the enclosure.

OK, time for some real pictures of the actual enclosure mounted on the ceiling. I
apologize for not having any pictures during the actual construction...I didn't
have my camera yet.

The side image shows an extra bit of felt glued to the back of the enclosure
near the projector. This prevents some of the reflections generated by
the prisms from getting out and reduces the light spill onto the walls. An
additional piece of felt is actually glued to the back of the box with just
a circular cutout for the projector lens. You can see the wing nuts on
the bottom of the enclosure box that are used to adjust the prism angles. You
can also see the wing nuts on the side which are used to attach the bracket
to the box, and the slots where the bracket is bolted to the white sled. If
you look closely, you can see the plexiglas that is epoxied to the top of
the prism. You can also see the black electrical tape that covers the
wide end of the prism (on the right side of the prism in the first image).

Here is the enclosure moved
to the side of the projector:

This gives a better view of the "sled" which is attached to the ceiling using
two drawer slides. To move the lens out of the way, I simply push the
entire lens assembly to the left. The ceiling is low enough that it is
easy for me to just reach up to move the lens. If you had a higher ceiling,
then you could attach a cord to the sled to drag it left or right. The
drawer stops prevent the sled from being moved too far in one direction or
the other. You can actually build the sled even better and can completely
hide the drawer slides even when the lens is moved out of the way. I
got my upside-down directions messed up, which is the only reason the drawer
slides show. But at least it makes the picture easier to understand.

Notice that the bracket is designed to allow the lens box to tilt up and down. A
slight tilt is needed to improve the geometry of the image. You will get
pin cushioning in the corners of the image, and by tilting the entire lens
you can minimize this geometry distortion and make it more uniform on the top
and bottom of the image. I actually zoom my image out just a bit so that
the pin cushioning is masked by the black felt around the outside edge of my
screen.

Performance Tests

Uniformity

When watching movies, you will be amazed at the performance of this lens. The
last Star Wars image shown above is an actual picture using this lens. However,
if you connect a computer to the projector and start playing with test images,
then you will learn a bit more about the potential short-comings of this DIY
lens. But before I discuss these details, I can't stress enough how minor
they really are. It's easy to get depressed when looking at test images. The
test images are useful to try and improve the design, but the overall result
of watching movies is much more positive than you might think looking at the
test images.

The first issue is the uniformity of the aspect ratio. Converting a
16:9 image to a 2.35:1 image requires a 133% stretch. However, if you
put a grid onto the screen and actually measure the rectangles, you will discover
that the rectangles in the center of the screen are 127%, and the rectangles
on the right and left edges of the screen are 142%. The perfect 133%
stretch only occurs in the left-center and right-center of the screen. It
is uniform from top to bottom. It is only left-to-right that shows the
aspect ration change.

Fortunately, the human eye is not sensitive to these kind of changes in aspect
ratio. Even when moving an animated circle around the screen, it is very
difficult to detect the aspect ratio changes to the circle as it moves left
and right on the screen from the normal viewing distance (about 10 ft from
the screen in my case, which is closer than most for a 124"x53" screen).

Also, according to reports from people who own commercial Anamorphic Lenses,
the commercial lenses suffer from the same aspect ratio non-uniformity.

Chromatic Aberration (CA)

A larger problem is the color problems at the edges of the screen. As
we know, the first prism is splitting the light beam into a rainbow. If
the second prism can't precisely undo this, then you won't get a white beam
on the screen. And in fact, it is impossible for a flat prism surface
to perfectly correct the rainbow from the first prism because the light beam
is actually a spherical wave and not a flat wave. So, as you move towards
the right and left edges of the screen, the light beam starts to get split
more into a rainbow. This gives a slightly colored edge to some objects. Here
are some very close up pictures of the test grid in the middle of the screen,
and then on the right edge of the screen:

Notice how the horizontal lines are still perfectly black in both images. However,
the vertical grid lines are black in the left image, but look like rainbows
in the right image. If you look closely, you will see that the black
line is blue and has a yellow edge on the right side of it. This makes
the image look blurry. Notice that you can actually see the DLP pixel
structure in these images, and that the rainbow is about two pixels wide. Now
here is a full view of the same grid on the entire screen:

(click for larger image)

Note that the geometry pin cushioning and the change in brightness across
the screen is caused by the digital camera and not the lens or projector. But
you can still tell that the left-most and right-most parts of the screen are
a bit blurrier than the center of the image.

This chromatic aberration can be reduced by using four prisms instead of
two. You essentially "double" each prism by putting another prism next
to it in the exact same orientation. Essentially you are making two prisms
with a larger internal angle. This allows you to reduce the angles of
the prisms relative to the light beam, resulting in less bending of the light. The
less the light beam is bent, then the smaller the rainbow on the edges.

At normal viewing distance, the eye cannot really detect the rainbow itself. It's
very hard to see any color edges on objects in movies. The only real
effect of this problem is that it makes the image on the left-most and right-most
parts of the screen slightly blurrier. While this is noticeable when
using a computer image, it really isn't very noticeable during movies. However,
this is certainly something to be improved in these lenses and something the
commercial lenses do a better job of correcting.

Reflections

As mentioned before, each time a beam of light hits a surface, it is both
refracted (bent) and reflected. In a dark theater, these reflections can
be quite visible and quite distracting if you don't do something to get rid
of them. Fortunately, the majority of reflections are at angles that
can be easily blocked by the enclosure. When you first play with the
prisms without an enclosure, however, you will be able to see all of the different
reflections around the side and back walls of your room.

Fortunately, all we need to worry about are reflections that end up on the
front screen. As it turns out, there is a single reflection from the
2-prism lens that can appear on the front screen. This reflection occurs
just as the beam of light is exiting the final surface of the final prism. The
light reflects internally within the prism, then reflects again on the first
surface of the same prism, resulting in a beam that hits the screen near the
first beam. Fortunately, since this is a double-reflection, it's intensity
is approximately 0.1% of the main beam intensity. Also, given the geometry
of the lens shown in this article, only the right-most part of the image is
reflected onto the left-most part of the screen. Given the low intensity,
you can only see this reflection when there is a bright object on the right
side of the screen and the left side of the screen is very dark. In some
cases, you might see a dim reflection of credits at the end of some movies,
since they are moving (making the reflection easier to detect) and are typically
bright white letters on a black background. However, this reflection
is so dim that I wasn't able to get a picture of it in my camera. And
most movies do not show this reflection at all. The best test case I
have for demonstrating the reflection is the very opening of the James Bond
Goldeneye movie, where the white spot light moves from left to right (the trademark
James Bond opening where the white spotlight turns red and then zooms into
the actual first action scene). When the white spotlight is on the right-most
part of the screen, you can see a dim reflection on the left side of the screen.

Using more prisms to reduce chromatic aberration unfortunately provides more
surfaces to generate reflections. A 4-prism lens has several other reflections
that can hit the screen. I find myself more sensitive to reflections
and less sensitive to chromatic aberration, so I prefer the 2-prism lens. But
you might want to play with four prisms to see what you think yourself.

Unfortunately, the only way to reduce this reflection is to coat the prisms
with some sort of non-reflective coating, without effecting the optical clarity
of the prism. Such coatings are very expensive and are something you
can expect on commercial lenses, but difficult to achieve for DIYers.

Conclusions

I cannot stress enough how immersive a constant-image-height (CIH) theater
can be. If
you can remember your first thrill at having a large "movie theater" in
your home, going to a 2.35:1 screen with CIH will give you that same thrill
all over again. I had thought for the past five years that just zooming
the projector was good enough (and it was certainly better than just
a 16:9 screen). But
using an Anamorphic Lens instead of just zooming the projector is simply an
amazing difference in picture quality. When I tell people about CIH,
I always warn them that it's almost a curse: once you have experienced a CIH
theater, then you never want to go back to a simple HDTV or 16:9 screen again.

Fortunately, thanks to many DIYers, it is possible to construct your own Anamorphic
Lens fairly easily and inexpensively. It has certainly been one of the
most satisfying projects I have ever done for my theater. I'll leave
you with an image that just seems to express my happiness with this lens.

Acknowledgements

I owe my thanks to many people who have contributed ideas to this project. Most
of these people have contributed to the very
long forum thread on the DIYAudio.com site. If you are serious about
a DIY Anamorphic Lens, then you own yourself to spend the hours it will take
to read this entire thread. Mostly
I want to thank Mark
Techer and his Aussiemorphic
Lens blog, not to mention his many early posts about building water and
oil prisms before the crystal wedges were discovered. Then I should thank
Steve Scherrer for his
discovery and post
of the U.S. source for these prisms, and for his ideas on
attaching screws to the prisms to make them adjustable. Also, thanks
to dvarma who posted a PDF link
to his description of an enclosure that used
a drawer slide to attach it to the ceiling, which was an inspiration to my
own enclosure bracket.

I wish the best of luck to anyone who attempts this project. Rather
than contact me directly, I encourage you to post questions and your own experiences
to the DIYAudio.com thread mentioned above so that you can get help from everyone
in that forum and help inspire others to built their own CIH Theater.